This variant is a little bit more easy to read and should be executed faster.

while (<DATA>) {
    my ( $left, $right ) = split /-1/, $_, 2;
    $left =~ s/ /,/g;
    print join '-1', $left, $right;
}

__DATA__
01 fines name 2222 -P sws -1 reee.tee rrt
02 fi si 2232 -P sqww -1 re.wqw ttf
02 fi si-1 2232 -P sqww -1 re.wqw ttf
[download]

Hi!

Your code is slower by about a factor of 3:

use Benchmark qw/:all :hireswallclock/;

my $DATA = '01 fines name 2222 -P sws -1 reee.tee rrt';

cmpthese(-10, {
    'rjt'       => q{ $_ = $DATA; s/ (?=.*\s?-1 )/,/g; },
    'pushtaev'  => q{
        $_ = $DATA;
        my ( $left, $right ) = split /-1/, $_, 2;
        $left =~ s/ /,/g;
        join '-1', $left, $right;
    },
});
[download]

              Rate pushtaev      rjt
pushtaev 1875006/s       --     -75%
rjt      7582716/s     304%       --
[download]

Regarding readability, it will depend who you ask in this case. In my experience, I tend to find a simple regex easier to read than tearing apart and piecing a solution back together with split, s///, and join.

Lastly, in your code, replacing this:

    print join '-1', $left, $right;

... with this:

    print $left, '-1', $right;

... resulted in a 32% speed improvement before I/O on your solution (still ~2.2x slower than the straight regex) and puts the terms in their natural order, which is probably more readable than the strange use of join with a delimiter to concatenate three strings.

I hope this is useful feedback to your feedback!